Zirconia membranes

Zirconia membranes on carbon supports were originally developed by Union Carbide. Ultrafiltration membranes are commercially available now under trade names like Ucarsep and Carbosep. Their outstanding quality is their high chemical resistance which allows steam sterilization and cleaning procedures in the pH range 0-14 at temperatures up to 80°C. These systems consist of a sintered carbon tube with an ultrafiltration layer of a metallic oxide, usually zirconia. Typical tube dimensions are 10 mm (outer diameter) with a wall thickness of 2 mm (Gerster and Veyre 1985). 

Guizard et al. (1986), Cot, Guizard and Larbot (1988) and Larbot et al. (1989) used a sol-gel method to prepare zirconia membrane top layers on an alumina support. The water necessary for the hydrolysis of the Zr-alkoxide was obtained from an esterification reaction. The complete hydrolysis was done at room temperature and resulted in a hydrated oxide. The precipitate was peptized with nitric or hydrochloric acid at pH 1.1 and the final sol 

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The advantage of sol-gel technology is the ability to produce a highly pure y-alumina and zirconia membrane at medium temperatures, about 700 °C, with a uniform pore size distribution in a thin film. However, the membrane is sensitive to heat treatment, resulting in cracking on the film layer. A successful crack-free product was produced, but it needed special care and time for suitable heat curing. Only y-alumina membrane have the disadvantage of poor chemical and thermal stability. 

The zirconia membrane was obtained in a unique manner. Figure 16.24 shows light micrographs of the zirconia-alumina membrane coated on the ceramic support. The non-unifomity and crater-filled surface of the ceramic support was covered by the zirconia-alumina membrane layer. Zirconia was mounted by very thin or nano-layers on the ceramic membrane. 

A thin zirconia membrane is sealed to a stainless cup of about one inch diameter (Figure 5.9). The stainless cup has a feed tube welded to it. The inside cavity is evacuated to about 5 psi and sealed off. One can calculate the leak rate by monitoring the pressure decay as a function of time (see Section 5.2.1). 

FIGURE 5.13 Reference electrode response in a zirconia membrane cell over five thermal cycles. 

Figure 3.6. Pore size distribution by mercury porosimetry of a two-layered zirconia membrane composite.

Similarly, impervious yttria-stabilized zirconia membranes doped with titania have been prepared by the electrochemical vapor deposition method [Hazbun, 1988]. Zirconium, yttrium and titanium chlorides in vapor form react with oxygen on the heated surface of a porous support tube in a reaction chamber at 1,100 to 1,300 C under controlled conditions. Membranes with a thickness of 2 to 60 pm have been made this way. The dopant, titania, is added to increase electron How of the resultant membrane and can be tailored to achieve the desired balance between ionic and electronic conductivity. Brinkman and Burggraaf [1995] also used electrochemical vapor deposition to grow thin, dense layers of zirconia/yttria/terbia membranes on porous ceramic supports. Depending on the deposition temperature, the growth of the membrane layer is limited by the bulk electrochemical transport or pore diffusion. 

Effect of heat treating temperature on mean pore diameters of alumina, titania and zirconia membranes... 

Figure 4.22 Molecular weight cutoff curve of a 5 nm pore diameter zirconia membrane using dextians of varying molecular weights as test molecules [Hsieh et al, 1991]...

T-irconia membranes. Along with alumina membranes, zirconia membranes have been a major driving force among various inorganic membranes for serving the separation... 

The year 1980 marked the entry of a new type of commercial ceramic membrane into the separation market. SPEC in France introduced a zirconia membrane on a porous carbon support called Carbosep. This was followed in 1984 by the introduction of alumina membranes on alumina supports, Membralox by Ceraver in France and Ceraflo by Norton in the U.S. With the advent of commercialization of these ceramic membranes in the eighties, the general interest level in inorganic membranes has been aroused to a historical high. Several companies involved in the gas diffusion processes were responsible for this upsurge of interest and applications. 

Although some inorganic membranes such as porous glass and dense palladium membranes have been commercially available for some time, the recent escalated commercial activities of inorganic membranes can be attributed to the availability of large-scale ceramic membranes of consistent quality. As indicated in Chapter 2, commercialization of alumina and zirconia membranes mostly has been the technical and marketing extensions of the development activities in gas diffusion membranes for the nuclear industry. 

There are a variety of porous inorganic membranes in the market today. Highlighted in Table 5.1 are selected major commercial inorganic membranes according to their material type. So far the most widely used inorganic membranes are alumina membranes, followed by zirconia membranes. Porous glass and metal (such as stainless steel and silver) membranes have also begun to attract attention. 

Although porous glass membranes have been around for some time, alumina membranes are finding more uses than other inorganic membranes. Zirconia membranes are also receiving much attention. Porous metal membranes are available and are used to less extent due to their unit costs. New inorganic membranes such as titania membranes are emerging. 

MUk protein standardizatiop (concentration of pasteurized skimmed milk). Milk protein standardization is designed to maintain the protein level in the milk constant all year round for automated cheese making. It basically involves concentration of pasteurized skimmed milk. It has been one of the major commercial successes in using inorganic membranes for food applications. In commercial production, microfiltration alpha-alumina and zirconia membranes with a pore diameter of 0.1 to 0.7 pm (mostly 0.2 pm) are used. Skimmed milk pasteurized at 70 C is typically concentrated to a volume concentration factor of 2 to 5 [Attia et al., 1988 van der Horst et al., 1994]. The volume concentration factor is the ratio of the initial feed volume to the retentate volume. Thus the higher the factor is, the more concentrated the product becomes. 

Milk protein standardization for continuous cheese making can also be done by ultrafiltration using ceramic membranes. Zirconia membranes with an average molecular weight cut-off (MWCO) of 70,000 daltons on carbon supports have been used for this purpose. The objective for this application is to concentrate either the whole volume of the milk to a volume concentration factor of 1.3 to 1.6 or just a fraction of the feed volume to a volume concentration factor of 3 to 4 followed by mixing the concentrate with raw milk to reduce the requirement of milk storage space [Merin and Daufin, 1989]. 

High-purity WPC (i.e., 70-95% proteins or total solids) can be produced by thermocalcic aggregation, followed by microfiltration, ultra filtration and diafiltration of whey proteins. Ultraflltration has been practiced since early 1970s. It appears that zirconia membranes on carbon supports with a MWCO of 10,000 to 20,000 daltons and zirconia membranes on alumina supports with a pore diameter of 0.05 to 0.1 pm are suitable for this purpose. A permeate flux of as high as 60 L/hr-m for processing acid whey to a protein content of 25 to 37% using a zirconia membrane with a MWCO of 10,000 daltons has been reported [Merin and Daufin, 1989]. 

Protein concentration. Concentration of proteins of several animal and plant products is practiced in food applications. Inorganic membranes have been used in many of these cases [Merin and Daufin, 1989]. For example, separation and concentration of egg white proteins from yolk using 20,000 dalton zirconia membranes increases the protein solids from 11-12% to 32-35%. Zirconia membranes with a MWCO of 70.000 daltons raises... 

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